The present disclosure generally relates to systems and methods for coating removal. More particularly, this disclosure is directed to a system and method that provides real-time compositional feedback to a laser ablation machine in order to identify when the ablation process has reached an optimal end point during a coating removal process.
Hot section components of turbomachines, including gas turbines employed for power generation and propulsion, are often protected by one or more coating layers, such as a thermal barrier coating (TBC), to reduce the temperature of the underlying component substrate and thereby prolong the service life of the component. Ceramic materials and particularly yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. Plasma spraying processes such as air plasma spraying (APS) yield noncolumnar coatings characterized by a degree of inhomogeneity and porosity, and have the advantages of relatively low equipment costs and ease of application. TBC's employed in the highest temperature regions of turbomachines are often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure. Similar columnar microstructures with a degree of porosity can be produced using other atomic and molecular vapor processes.
To be effective, a TBC must strongly adhere to the component and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion (CTE) between ceramic materials and the substrates they protect, which are typically superalloys, though ceramic matrix composite (CMC) materials are also used. An oxidation-resistant bond coat is often employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of a diffusion aluminide coating or an overlay coating such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium, a rare earth element, or a reactive element). During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during turbine operation, these bond coats form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat.
During the manufacture and/or maintenance of the turbine component and deposition of a one or more layers of a protective coating material thereon, portions of the component intended to be free of any coating material may become at least partially covered during the coating process. In addition, the service life of these one or more protective coating layers is typically limited by a spallation event driven by bond coat oxidation, increased interfacial stresses, and the resulting thermal fatigue. In either situation, removal of the protective coating layer is required.
In current manufacturing processes laser ablation may be utilized for coating removal by rapidly scanning a laser beam across a coated surface with multiple passes to remove a desired material thickness (such as TBC coating removal on the LEAP S1B trailing edge cooling slot). However, the number of passes that are required to completely remove a desired layer, while not breaking into the sub layers, is determined empirically by trial and error or with a gage to measure the physical material removed (depth or mass). For applications such as TBC coating removal on LEAP S1B trailing edge cooling slots, tight tolerances necessitate optimized laser ablation to avoid tedious manual inspection and then rework which is time consuming and incurs high manufacturing cost.
Typical laser ablation coating removal processes employ a laser pulse to remove materials. Laser induced breakdown spectroscopy (LIBS), as an analytical method, employs the same laser pulse. As known in the art, LIBS entails projecting a pulsed laser beam onto a material at a power density sufficient to vaporize (ablate) a small portion of the material and generate a luminous plasma that contains the characteristic atomic emission lines of elements within the material, which are then collected for spectral analysis. LIBS systems employ the use of real-time measurements to enable this spectral analysis, facilitating real-time monitoring and control.
Accordingly there is an ongoing need for more convenient and less obtrusive techniques to remove coating materials. It would be desirable to provide a robust removal process that is operational regardless of coating thickness variation, avoids sublayer damage, and minimizes tedious manual inspection of reworked-parts. It would also be desirable to provide a system and method that integrates real-time measurement with an existing laser removal process thus enabling real-time monitoring and control.